Study of shipping routes maps delivery of invasive organisms
When giant container ships sail into major ports like Los Angeles and Long Beach, it’s not just clothing and cars that they deliver. They also carry critters.
The specimens — microscopic algae cells or larger castaways, such as eggs of fish or crustaceans — float about in the thousands of tons of water the boats use as ballast. When the ships dump their ballast at port, the species can establish a foothold in foreign lands, often with detrimental consequences to native wildlife.
But soon ports may be able to mitigate some of that harm by predicting where invasive species are likely to arrive. In a study published Sunday in the journal Ecology Letters, a team of European scientists mined millions of signals transmitted from ships entering harbors and used the information to figure out which of the world’s ports are most vulnerable to an influx of invasive species.
Los Angeles and Long Beach, which together receive about 40% of the container imports entering the U.S., both made the team’s list of the 20 highest-risk “hot spots.” So did eight ports in Southeast Asia, five in the Middle East, the port of New York and New Jersey, and the Panama Canal.
The impact of invasive species carried by ships can be severe. In 2008, researchers at Notre Dame and the University of Wyoming estimated that the cost to the Great Lakes region alone was at least $200 million — every year.
Some organisms travel on boats’ hulls, but most are thought to make the trip in the ballast water carried on ships to balance their loads.
In the Black Sea, an invasive jellyfish called Mnemiopsis was probably jettisoned in ballast water carried from the U.S. The beasts crowded out other sea life and devastated the region’s fishing and tourism industries. In California, the Department of Fish and Wildlife has suggested that two pesky beasts plaguing the state’s waters — quagga mussels and zebra mussels, which hail from the Ukraine and Russia and clog pipes and other infrastructure — could also have made their journeys in ballast water.
The new study doesn’t examine the impact of any one invader. Rather, the researchers attempted an analysis that could predict an invasion of any beasts from any locations to any destinations.
The first step was to pull together a record of what ships traveled where, and when — a task that required a lot of detective work. “The port authorities said to go to the shipping companies, and the shipping companies said to go to the port authorities,” said Bernd Blasius of the Institute for Chemistry and Biology of the Marine Environment at Carl von Ossietzky University in Oldenburg, Germany, who led the research team.
The researchers wound up assembling their model of the global shipping route network using data from the mandatory Automatic Identification System, which reports ships’ approaches to coastal stations. Their network included 32,511 ships that made 2,892,523 journeys to 1,469 ports in 2007 and 2008.
Next, the researchers combined the shipping information with other factors known to affect whether a particular species was likely to survive in distant waters. They looked at ship sizes — the larger the boat, the better the chance it will harbor invasive species — as well as how well ocean salinity and temperature matched up across ports, among other details.
Analyzing all of the variables together, they were able to figure out that the intermediate distances — trade routes of about 5,000 to 6,000 miles — posed the greatest threat.
Most of the hitchhiking organisms were transported to nearby waters — “but those species are already there,” Blasius said. On the flip side, castaways from far-off areas could not survive the travel times required to make longer journeys, he said.
While higher-volume ports were at higher risk, some heavily trafficked areas, such as Europe’s North Sea, dodged the bullet because of environmental conditions. In the case of Northern Europe, waters are too cold for tropical species to survive.
Comparing their model’s output to data collected in four other studies — including one documenting invasive species in San Francisco Bay — the researchers discovered that their predictions matched well with reality in the water.
“We were astonished,” Blasius said.
The team also analyzed whether treating ballast water — a remedy proposed by the International Maritime Organization — would affect species invasion. They calculated that modest efforts, such as filtering some ballast water or cleaning it with ozone or other chemicals, could yield substantial results. Cleaning the water by 25% in all ships at the 10 highest-risk ports would reduce invasion risk across the entire network by 24.8%, they estimated.
It “shows the value of treating ballast water,” said Hugh MacIsaac, a professor at the University of Windsor in Ontario who was not involved in the work.
UC Davis ecologist Alan Hastings, an expert in invasive species who was also not involved in the study, said that building detailed shipping routes into the analysis was “a huge deal” that would improve scientists’ understanding of the entire chain of events that causes a species to invade.
Having a better way to predict hot spots would allow authorities to focus expensive mitigation efforts on the highest-risk ports or vessels, he said.
“If we can get a better understanding of the biggest risk, we can get more bang for our buck in terms of preventing or reducing the impact of invasions,” Hastings said.
In the future, Blasius and his colleagues plan to study diseases that might be transported in ballast water, such as cholera. The team would also like to assess how new trade routes through the Arctic could affect native animals and other species.
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